The invention concerns a fixing element for fixing a first bone fragment to a corresponding second bone fragment, as in an ankle joint fracture.
A problem which frequently arises in the case of fractures caused by over-stressing is that relatively small exposed bone regions are detached from the rest of the bone. Thus for example ankle fractures represent the most frequent break injury to the lower extremity. In that case, over-stressing in the region of the upper ankle joint means that the Inner ankle Is detached from the tibia and/or the outer ankle is detached from the fibula. In order to restore the function of the ankle joint, the ankle fragments must be fixed to the associated bone fragment until the break heals. Bone screws and the like are still frequently used for that purpose. As however generally particularly long and/or a plurality of bone screws are required for adequate stabilisation of the bone fragments by virtue of the slenderness of the bone screws, the fixing elements of the general kind set forth which are easier and quicker to implant are increasingly gaining in importance.
European patent application EP 0 409 364 A2 discloses a fixing element of the general kind set forth, in which at its proximal end, that is to say the end towards the surgeon, the fixing body has a step which permits the fixing body to be inserted into the receiving bores in the bone fragments, only as far as a given depth. The fixing body which in the initial condition has a conical main body with anchoring elements arranged thereon is spread open in particular at its distal end, by driving in the spreading body, to such an extent that the main body assumes a substantially cylindrical contour. The anchoring elements of the spread-open regions in that case penetrate into the surrounding bone and thus serve to provide for additional fixing.
That fixing element however suffers from the disadvantage that the presence of the step at the proximal end means that It Is only suitable for fixing bone fragments up to a given maximum dimension in the longitudinal direction of the fixing body. If that maximum dimension is exceeded, there is no longer any guarantee that the first bone fragment is adequately fixed to the second bone fragment. As the distal end of the fixing body is spread open In the spongiosa which is generally of low strength, it can tear out of the second bone fragment with a correspondingly short depth of penetration or in a situation involving locally reduced strength of the bone tissue. It is precisely a locally reduced level of strength in the bone tissue that can only be detected with difficulty during the operation, so that, with a depth of penetration which is possibly still sufficient in a normal situation, it can nonetheless tear out.
If the step at the proximal end of the fixing body is omitted, the fixing body can admittedly penetrate sufficiently far into the second bone fragment, irrespective of the dimension of the first bone fragment. As the spreading body at the proximal end is of a smaller diameter than the cavity, no expansion of the fixing body takes place immediately at the proximal end. Rather, expansion of the fixing body increases slowly in the direction towards the distal end so that the absence of the step means that there is precisely no guarantee of secure fixing of relatively thin first bone fragments.
The known fixing element in its respective configurations is therefore only limitedly suitable for fixing bone fragments of the most widely varying dimensions so that in practice an expensive set of fixing elements in a plurality of different dimensions is required for the respective situations of use.
U.S. Pat. No. 5,713,904 B1 to Errico, et al. (Feb. 3, 1998), discloses a fixing element of the general kind set forth, in which an integral tubular fixing body is provided at both ends with longitudinal slots so that, upon the insertion of a suitable spreading body into its internal cavity, it is spread open in a cup-shaped configuration at both ends while it remains undeformed in its central region. The fixing body in that case is provided with a male screwthread by which of which it can be screwed into the bore in the bone.
That fixing element has the disadvantage however that, in its middle region, the contact force between the fixing body and the surrounding bone is determined by the fit therebetween. In that respect, it is precisely in the comparatively soft spongiosa of a bone that a greater or lesser deviation of the bore diameter from its target diameter can occur, due to a minor deflection with the drill from the target axis for the bore. Depending on the over-size of the bore with respect to the fixing body, the situation can then involve a connection which is movable to a correspondingly greater or lesser degree transversely with respect to the longitudinal direction of the fixing body, if the central region of the fixing body is precisely in the region of the join between two bone fragments which are to be fixed to each other.
Therefore the object of the present invention is to provide a fixing element of the general kind set forth, which does not suffer from the above-stated disadvantages or which suffers therefrom at least to a lesser degree, and which in particular can be put to flexible use and ensures reliable fixing.
Based on a fixing element as set forth in the appending claims, that object is attained by the features recited therein.
The invention involves the technical teaching that a fixing element which can be put to flexible use is acquired if the fixing body is adapted to be introduced completely into the bores and by proximal insertion of the spreading body into the cavity can be spread open transversely with respect to its longitudinal direction by a wedge action substantially over its entire length for connection to the respective bone fragment. That ensures on the one hand that, irrespective of the thickness of the first bone fragment, the fixing body can be moved into a position in which the second portion of the fixing body is sufficiently deeply inserted into the second bone fragment in order to ensure fixing of the first bone fragment, which is adequate in any situation. In that respect, spreading the fixing body open in the region of its proximal end ensures that, even when dealing with particularly thin first bone fragments, reliable fixing is guaranteed with the fixing element according to the invention.
Reliable fixing is further achieved in that, in the spread condition, that is to say after substantially complete insertion of the spreading body into the cavity, the fixing body at the distal end of the second portion is of a larger dimension transversely with respect to its longitudinal direction than at the proximal end of the second portion. The fixing body is thus enlarged conically or bell-like at its distal end in the second bone fragment in which it is generally surrounded by spongiosa of lower strength, thereby achieving a positively locking engagement over a large area. That ensures secure anchoring even if the surrounding bone material is of a possibly reduced level of strength, insofar as it applies the anchoring loads uniformly to a larger volume of bone. That effectively prevents the fixing body from being torn out of the second bone fragment.
The operative surfaces in the region of the cavity and on the spreading element, which are responsible for spreading open the fixing body, can be designed in many known ways. Thus to provide that the second portion of the fixing body is expanded in a conical or bell-like manner, it is only necessary for the transverse dimension of the cavity to decrease towards the distal end. In that respect, the transverse dimension of the spreading element can remain constant or also decrease towards the distal end, in which case the decrease per unit of length must then be less than the decrease in the transverse dimension of the cavity towards the distal end.
The fixing body can consist of a single body which, for greater ease of being spread open in the regions which are to be spread apart, is provided with a respective one or preferably a plurality of longitudinal slots or the like. If the fixing body has a plurality of those longitudinal slots, the spreading action which varies in the peripheral direction additionally provides for non-rotationally securing the fixing body about the longitudinal axis thereof. Preferably, the fixing body comprises at least two body portions which are in adjoining relationship In the peripheral direction and which are connected together movably sufficiently for being spread open. In that respect, the connection only has to be so firm that, upon being introduced into the bores and upon Introduction of the spreading element into the cavity, the body portions are held relative to each other in the longitudinal and peripheral directions, at least until the spreading element and the wall of the bore take over that function in the spreading procedure. In this case, the spreading effect which varies in the peripheral direction additionally provides for non-rotationally securing the fixing element about the longitudinal axis thereof.
The body portions can be connected by way of suitably thin leg-like bridging regions which are connected to the body portions and which, when the fixing element Is spread open, are correspondingly slightly stretched, unfolded or torn open. It is however also possible to provide on each of the body portions respective guide elements which co-operate with the corresponding guide elements of the adjoining body portion and which thus hold the body portions relative to each other in the longitudinal and peripheral directions. Thus for example projections with one or more substantially tangentially extending guide surfaces on one body portion can co-operate with guide grooves of a corresponding configuration on the adjoining body portion.
In preferred embodiments of the invention, the operative surfaces of the fixing body and the spreading body, which co-operate to spread the fixing body open, are of such a configuration that spreading of the second portion begins at the distal end of the second portion. That ensures that a distribution of stress which is as uniform as possible is achieved in the spreading operation in the second portion and thus local stress peaks are substantially avoided both in the fixing body and also in the bone.
The operative surfaces In the region of the cavity and on the spreading element, which are responsible for spreading open the fixing body, can be designed for that purpose in many known ways. Thus, for conical or bell-like expansion of the second portion of the fixing body, which begins at the distal end, it is only necessary for the transverse dimensions of the cavity and the spreading body to decrease towards the distal end, in which case the transverse dimension of the cavity and of the spreading body substantially correspond to each other at the distal end and the reduction in the transverse dimension of the spreading body per unit of length must be less than the reduction in the transverse dimension of the cavity towards the distal end.
In that respect, it is particularly advantageous if the operative surfaces are such that at least a first part of the first portion is spread open before the second portion is spread open. In that way, the reduction in the size of the second portion, as a consequence of its conical or bell-shaped spreading beginning at the distal end, is transmitted by way of the first part, which is already spread open, of the first portion, to the first bone fragment, so that the latter is pressed against the second bone fragment in a manner which is advantageous in terms of the healing process. In that respect, it is further preferable for the first part to be arranged in the region of the proximal end of the first portion, so that this advantage can also be enjoyed in the case of a thin first bone fragment.
In particularly advantageous developments of the fixing element according to the invention the first portion is pivotably connected at its distal end by way of at least one web or leg element to the proximal end of the second portion. In that case, the fixing body and the spreading body are of such a configuration that, upon insertion of the spreading body, the distal end of the first portion is substantially completely spread open before in succession in a first step a part of the second portion is spread open and in a second step the proximal end of the second portion is spread open. Alternatively, the fixing body and the spreading body can be of such a configuration that upon insertion of the spreading body the proximal end of the second portion is substantially completely spread open before in succession in a first step a part of the first portion is spread open and in a second step the distal end of the first portion is spread open. In addition, the configuration and arrangement of the leg element is such that the longitudinal spacing between the first and second portions is reduced during the second step.
That provides in a simple manner that the reduction in the longitudinal spacing between the first and second portions is transmitted to the first and second bone fragments, by way of the regions of the first and second portions respectively which are already connected to the respective bone fragment by virtue of being spread open when the reduction in length is initiated. In that way, the bone fragments are not only firmly fixed relative to each other but they are also pressed against each other in a manner which is advantageous in terms of the healing process.
In that respect, the leg element or elements can be simply arranged on the two portions in such a way that their angle of inclination relative to the longitudinal axis of the fixing body increases during the second step, thereby directly affording a corresponding reduction in the longitudinal spacing between the first and second portions.
In advantageous variants of the invention the fixing body, after insertion of the spreading body into the cavity, is of a larger dimension transversely with respect to its longitudinal direction at the proximal end of the first portion than at the distal end of the first portion. The conical or bell-shaped expansion achieved In that way at the proximal end of the fixing body promotes the careful application of load to the bone tissue of the bone fragment in question, in the manner which has already been described hereinbefore in respect of the distal end.
The fixing element according to the invention is distinguished in that the fixing body is adapted to spread open substantially over the entire length thereof. That ensures an appropriately distributed application of load to the bone tissue, which is adapted to the strength conditions thereof.
In particularly advantageous variants of the fixing element according to the invention the first portion in the non-spread condition is of a larger dimension transversely with respect to its longitudinal direction than the second portion. In that case, optimum positioning of the fixing body is guaranteed in a simple and reliable manner insofar as the second bone fragment is bored in the preparatory operation with a correspondingly smaller diameter than the first bone fragment. The resulting step in the region of the fracture gap then forms an abutment for the fixing body which accordingly, without further aids, can be introduced into the second bone fragment to the optimum depth of penetration, irrespective of the thickness of the first bone fragment.
The fixing element according to the invention preferably has for optimum fixing in the bone tissue at the surface of the fixing body which is towards the bone, projections which are intended to penetrate into the bone. The projections can be designed in many known ways. For example, they can be of a tooth-like or pin-like or spike-like or other configuration both In the longitudinal direction and alternatively or additionally in the peripheral direction, in order to provide for barb-like hooking engagement into the bone tissue. In that case they can be arranged to extend around the fixing body at the periphery thereof for example in rings or in a screwthread-like configuration.
It will be appreciated that the components of the fixing element according to the invention comprise biocompatible materials. Particularly advantageous variants of the fixing element according to the Invention provide that at least the fixing body comprises a bioresorbable material so that there is no need for subsequent explantation. It will be appreciated that preferably all components of the fixing element are made up of such bioresorbable materials in order entirely to avoid subsequent explantation of individual components. It will be appreciated that this can also be rendered unnecessary by virtue of using materials with long-term biocompatibility, for example for the spreading body. All biocompatible or bioresorbable materials with a level of time-dependent creep strength which is adequate for fixing can be used as the materials, the fixing body preferably comprising a polylactide. That is further preferably reinforced in regions involving an increased tensile loading by tension-resistant, in particular bioresorbable fibers and/or fiber cloth. In that case, it is possible to use for example tension-resistant materials as are used for surgical suture materials. An example in that respect is bioresorbable polyglactide.